“Hard water” is the term used to describe water with high mineral content. Hardness is generally characterized by a high concentration of multivalent cations, such as the divalent cations Ca2+ and Mg2+.
Water softening is commonly used to reduce hard water's adverse effects. Hard water can pose serious problems in industrial settings, causing costly breakdowns in boilers, cooling towers, and other equipment that handles water. Many industrial unit operations and unit processes require near-complete removal of hardness to minimize scale in heat transfer equipment, fouling in membranes and use of high concentrations of sequestering chemicals in cooling and wash water. In domestic settings, hard water often results in non-foaming soap water and deposition of lime scale in kettles and water heaters. Households in the central and southwestern U.S., which are generally supplied with ground water, require hardness removal or softening.
Lime softening and traditional cation exchange are the most commonly used processes for hardness removal. Lime softening uses Ca(OH)2 to increase pH, precipitate carbonates, and remove alkalinity. The process is operationally complex and generates large volumes of solid waste. Traditional ion exchange hardness removal processes use a strong acid cation exchanger in sodium form. Calcium, magnesium and other polyvalent cations contributing to the hardness are exchanged on an equivalent basis with sodium in the resin phase. Exhausted resins are typically regenerated with brine solution (10-15% sodium chloride). This process is inefficient, and the requirement of high concentrations of brine solution results in discharge of nearly 60-80% of the brine used.
The recently developed shallow shell technology improves kinetics of the ion exchange regeneration process and reduces brine consumption. However, the traditional cation exchange water softeners still have a salt discharge problem due to unfavorable equilibrium, i.e., 3-8 equivalents of Na+ are required to desorb one equivalent of hardness. Further, Na+ ions that exchange with hardness (i.e., Ca2+ or Mg2+) are added into treated water at the same equivalent amount to the hardness being exchanged, and this create a problem for people suffering from hypertension. Waste brine solutions from the spent regenerant cause high total dissolved solids (TDS) in aquatic systems, hurting aquatic life and damaging septic systems and water reclamation facilities. In response, California, Texas and Florida have passed laws requiring salt-free regeneration of water softeners.
There is a need in the art for a clean technology that eliminates the use of high concentrations of sodium salts in water softening procedures. Such technology should yield minimal amounts of salt waste and allow for the economical regeneration of the exchange medium.